The proposal describes a five-year training program with the purpose of establishing a career as an NIH funded independent investigator in reproductive medicine and infertility. Dr. Williams has a longstanding interest in and commitment to reproductive biology and women's health and has a strong track record in molecular biology and reproductive medicine. He earned his PhD in Molecular Biology in the laboratory of Dr. Paul Wassarman, where he studied the mammalian oocyte. He completed residency training in obstetrics and gynecology at the Brigham and Women's Hospital/Massachusetts General Hospital and is in the last year of fellowship training in reproductive endocrinology and infertility at Weill-Cornell Medical Center. Over the course of this award period he will expand his proficiency in genomics, RNA biology, assay design and computational biology with the overarching goal of elucidating the effect of transposons on oocytes and fertility. The training program has been designed to ensure command of RNA and transposon biology as applied to female reproduction. Dr. Thomas Tuschl, a pioneer in the field of RNA biology, will mentor the principal investigator's scientific development. He is Professor of Biochemistry at The Rockefeller University and has trained and guided many postdoctoral fellows to independent research positions. Training activities will include instruction in assay design and lectures in RNA molecular biology and computational biology. A scientific advisory committee of exceptional basic and physician-scientists will provide ongoing feedback. Transposons are mobile genetic elements comprising nearly half of the human genome. Their replication can cause genetic damage and disease and must be suppressed in the oocyte. In flies and worms, a special class of small RNA molecules (piRNA) and proteins (PIWI) suppress transposons. The long-term goal of the study is to investigate the mechanism by which mammalian oocytes suppress transposons and the effect of transposons on female fertility. The central hypothesis is that replication of transposons in mammals can cause miscarriage and is normally suppressed by piRNA and PIWI proteins. The hypothesis will be tested by pursuing three specific aims, namely: (1) to determine whether human ovaries contains piRNA and PIWI proteins (2) to assess the phenotype of the PIWI knockout in the female mouse (3) to determine the rate of transposon replication in normal pregnancies and miscarriage. The approach is innovative because it proposes a new etiology of infertility and involves developing assay utilizing whole-genome analysis with second- generation DNA sequencing. The research is significant because it will explain how the oocyte protects its genome from transposons and provides a new avenue for fertility diagnosis and treatment. Dr. Tuschl's laboratory at Rockefeller University is an ideal setting for intensive training in a set of highly specialized scientific skills required to address this complex area of reproductive biology and prepare the principal investigator for an academic career in reproductive biology.